This invention relates to the pretreatment of cellulose fibres under pressure with activating agents by which the fibres become more reactive in subsequent acetylation, followed by hydrolysis and precipitation to produce a high quality product (cellulose diacetate).
Cellulose acetylation is a heterogeneous reaction between the cellulose fibres and acetic anhydride in the presence of a catalyst, e.g., sulphuric acid. The course of the reaction is largely controlled by the diffusion rates of reactant and catalyst into the cellulose fibres after the initial surface esterification. Prior to the acetylation, a pretreatment or chemical activation is usually required, which swells the cellulose fibres and increases the accessibility of the cellulose hydroxyl groups to the acetylating agents.
Filterability of cellulose acetate solution is closely related with the formation of gel particles resulting from the presence of insufficiently activated fibre parts. A good filterability of cellulose acetate solution at a high viscosity level is required for the manufacture of high quality products of cellulose acetate.
Many attempts have been made to examine the activating effects of various pretreating agents (e.g., water, lower fatty acids, methanol, benzene, formamide, pyridine, etc.) and catalysts (e.g., methanesulfonic acid, perchloric acid and sulphuric acid).
According to existing technologies, pretreatment of cellulose is routinely made with acetic acid and a small amount of water. The presence of water accelerates the swelling of the cellulose fibre by acetic acid. The small size and polarity of water molecule can rapidly diffuse into the fibres and disrupt strained secondary valence bonds. This pretreatment method typically comprises a multi-stage process that usually requires special equipment in the following steps: (1) slurrying the cellulose in an acetic acid/water solution; (2) extracting water from the cellulose slurry by a glacial acetic acid wash and centrifuging to remove as much water as possible; and (3) fluffing and uniformly blending the cellulose with a sulphuric acid catalyst prior to entering the acetylation reactor. It may be noted that the viscosity of cellulose is sharply decreased when sulphuric acid catalyst is used even at a low level of less than 2% in the pretreatment step.
Despite such multi-step pretreatment, a small percentage of relatively inactive cellulose is always present. The poorly activated cellulose does not completely convert to desired cellulose triester in the acetylation step. The result is a turbid acetate solution. This problem is particularly acute in using wood pulp with relatively high degree of impurities such as high hemicellulose content.
U.S. Pat. Nos. 4,439,605 and 2,923,706 (and Canadian Patent No. 565,099) teach the minimization of insufficiently activated fibres by acetylation in the higher temperature range of from about 50 deg. to 85 deg. Celsius (or up to the boiling point of acetic acid), leading to the reduce formation of gel particles in cellulose triester. However,these approaches add substantially to the complexity of heating in the acetylation step.
U.S. Pat. Nos. 4,439,605 and 3,767,642 teach that after the complete neutralization of the sulphuric acid catalyst, the hydrolysis is conducted by injecting steam into the reaction mixture to reach higher temperature (up to about 160 deg. Celsius) and pressure (up to about 85 psig) for approximately 2 hours. However, this method usually results in an undesirable yellowish product.